Being a kind of important intermediates in organic reactions, isocyanates have a very wide application in industry, agriculture, medication and the like. This kind of compounds is broadly utilized in synthesizes of polyisocyanates, polyurethanes, polyureas, polymeric adhensives, insecticides, herbicides and the like.
Both aliphatic diisocyanates and aromatic diisocyanates are very useful industrial raw materials. For example, as an important raw material for preparing polyurethanes, aromatic 4,4-diphenylmethane diisocyanate (MDI) is widely applied in the preparations of microporous elastomers, thermoplastic elastomers, casting elastomers, leatheroids, synthetic leathers, adhesives, coatings, sealing agents and the like. As an industrial raw material, 2,4/2,6-toluene diisocyanate (TDI) is widely applied in the productions of polyurethane foaming plastics, polyurethane elastomers and millable elastomers, polyurethane coatings, polyurethane adhesives, polyurethane water-proof materials, detergents, thickening agent, antioxidants and the like. As a raw material for advanced polyurethanes, 1,5-naphthalene diisocyanate (NDI) can be used to manufacture polyurethane elastomers with high elasticity and high hardness. NDI polyurethane elastomers are widely used for automobile shock absorbers, forklift wheel bearings, printing and weaving rubber rollers, rubber scrapers, bridge-building buffers, entertainments or the like.
After being exposed outside, aliphatic and alicyclic diisocyanates do not get yellow, and therefore, they are also referred to as non-yellowing isocyanates. For example, 1,6-hexane diisocyanate (HDI) is mainly used for advanced coatings and refinishing paints for automobiles, OEM coatings and refinishing paints for airplanes, anticorrosive coatings, wood furniture paints, paints for enamel-insulated wires, refinishing paints for trains, polyurethane sizing agents with good light stability and rocket propellant additives or the like. 3-isocyanatomethyl-3,5,5-trimethylcyclohexyl isocyanate (isophorone diisocyanate) (IPDI) is mainly used to prepare aliphatic polyisocyanate derivatives, such as, polymers, trimers, adducts, which are used for excellent weather-proof polyurethane coatings, as well as weaving, PU leathers and many plastic products widely. 4,4-dicyclohexylmethane diisocyanate (HMDI) is mainly used for polyurethane foaming plastics, polyurethane elastomers and millable elastomers, polyurethane coatings, polyurethane adhesives, anticorrosive coatings, wood furniture paints or the like.
At present, isocyanates are mainly prepared by the reaction of the corresponding amine compounds and phosgene. Phosgene is a virulent compound, and additionally, a great deal of strong corrosive hydrogen chloride will be produced during the reaction. Therefore, it will result in apparatus corrosion and phosgene leakage which cause environment pollution and personnel injury.
With the increasing aggravation of environment pollution in the world, all countries implement continuously environment protecting measures in a manner of lows by force to control the usage and discharge of the poisonous harmful substances. The research and development for a clean productive technique of isocyanate chemicals by non-phosgene preparation has become an attractive area for the scientific institutions and chemical factories. The preparation of isocyanate chemicals by non-phosgene method is of benefit to the environment protection, and additionally, as there is no chlorine in the production medium, products with higher quality can be produced.
During the past 20 years or more, in order to find a safe, cheap, environment-friendly synthetic method for isocyanates, the people have carried out many researching work and found several routes for prepare isocyanates by non-phosgene method. In all of these, a route with the most industrialized perspective is a method of performing a carbonylation reaction by using an organic amine as raw material and taking dialkyl carbonate or small molecule alkyl urethane as carbonylation agent to synthesize the corresponding urethane, and then thermal cracking the urethane to obtain the corresponding isocyanate and alcohol.
The processing reaction condition for synthesizing the corresponding urethane from the organic amine and the carbonylation agent is relatively mild and easy to be realized. However, the thermal cracking process relates to higher temperature and is generally carried out under negative pressure. Additionally, isocyanates have relatively active chemical property and are easy to perform side reactions. Therefore, it is a critical and difficult process in the route for preparing isocyanates by a non-phosgene synthesis. The thermal cracking process is often attended by many side reactions, which will not only reduce the yield, but also block the reactors and other apparatuses. At the same time, as the vapor-phase thermal cracking has high reaction temperature (350-450° C.), many side reactions and low yield, the value of industrial application thereof is reduced. However, the liquid-phase thermal cracking with lower reaction temperature (200-350° C.) and higher yield has become a researching focus for numerous scientists gradually.
In U.S. Pat. No. 3,962,302, TDI was synthesized by thermal cracking toluene diethylurethane under nitrogen atmosphere at 250° C. in the case of taking cetane as solvent. The yield was 83.4%. In U.S. Pat. No. 4,294,774, Henson et al. prepared MDI with a yield of 46% when N,N-dimethylaniline was used as solvent and catalyst. In U.S. Pat. No. 4,349,484, Merger et al. prepared MDI with a yield of 76.5% by decomposing diphenylmethane dimethylurethane at high pressure and high temperature (310° C.), wherein decylbenzene was taken as solvent and filling zinc scraps were taken as catalyst. The above reactions have the following disadvantages of: (1) the reaction performed in the kettle reactor has a low yield; (2) under the condition of relatively high temperature and negative pressure, the high boiling solvents have the problems of decomposition, loss and the like; (3) as the alcohols produced cannot be separated in time, the alcohols tend to react with the isocyanates; (4) as isocyanates have active chemical properties, they tend to perform the side reactions of polymerization and the like.
The known researching results indicate that during the process of liquid-phase thermal cracking, the choice of solvents is critical in addition to the choice of appropriate catalysts and reactors. However, as the media in the thermal cracking process, the existing high boiling molecule type solvents have some formidable limitations.
In the previous researching work of the inventors (Youquan Deng, Xiaoguang Guo, Chinese invention patent application No.: 200610105297.3), 1,6-hexane diisocyanate was prepared by thermal cracking hexamethylene dimethylurethane in liquid phase in the case of using a supported solid catalyst (the catalyst was small spheres with a diameter of 2-5 millimeters) and an ionic liquid multiphase catalytic system. Comparing with the previous thermal cracking methods based on high boiling point molecule type solvents, the thermal cracking method based on ionic liquids have the advantages of less solvent addition, lower temperature of thermal cracking reaction, less by-products, higher thermal stability of ionic liquids, and reutilization and the like. However, there are also some disadvantages for above mentioned supported solid catalyst+ionic liquid multiphase catalytic system. For example, in the case of using a fixed bed reactor filled with a supported metal oxide solid catalyst, the ionic liquid containing reactants has relatively large flowing resistance and is difficult to be circulated, and therefore, the reaction efficiency thereof is lower and the reaction device is complex relatively. The amount of the supported catalyst charged is relatively large and the active components are easy to be lost, and therefore, the life of the catalyst is shorter which goes against the industrial application, and the universality of the catalyst system is lower.